chapter 2 – overvie · arq: selective repeat. tzi – fb 1 – kommunikationsnetze andreas...

TZI – FB 1 – Kommunikationsnetze Andreas Könsgen – Summer Term 2013

- 1 -

Chapter 2 – Overview• Part 1 (last week)

– Digital Transmission System– Frequencies, Spectrum Allocation– Radio Propagation and Radio Channels

• Part 2 (today)– Modulation, Coding, Error Correction

• Part 3 (next week)– Capacity limits– Duplexing schemes– Media Access Protocols


- 2 -

Modulation• Modification of the amplitude, phase or frequency of one or more sine-

shaped carriers• Combination between amplitude and phase modulation also possible• Analogue modulation schemes for transmission of continuous signals

– AM, FM, PM• Digital modulation schemes for transmission of discrete signals

– Binary Phase Shift Keying: change between two different phases– Quadrature Amplitude Modulation with 16 states (16QAM): 16

combinations of different amplitudesand phases


- 3 -

Digital Modulation Schemes• Modulation of digital signals known as Shift Keying• Amplitude Shift Keying (ASK):

– very simple– low bandwidth requirements– very susceptible to interference

• Frequency Shift Keying (FSK):– needs larger bandwidth

• Phase Shift Keying (PSK):– more complex– robust against interference

1 0 1

t

1 0 1

t

1 0 1

t


- 4 -

Advanced Phase Shift Keying• BPSK (Binary Phase Shift Keying):

– bit value 0: sine wave– bit value 1: inverted sine wave– very simple PSK– low spectral efficiency– robust, used e.g. in satellite systems

• QPSK (Quadrature Phase Shift Keying):– 2 bits coded as one symbol– symbol determines shift of sine wave– needs less bandwidth compared to

BPSK– more complex

• Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS)

11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t


- 5 -

Quadrature Amplitude Modulation

0000

0001

0011

1000

Q

I

0010

φ

a

• Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation

• it is possible to code n bits using one symbol• 2n discrete levels, n=2 identical to QPSK• bit error rate increases with n, but less errors compared to

comparable PSK scheme• Example: 16-QAM (4 bits = 1 symbol)• Symbols 0011 and 0001 have the

same phase φ, but different amplitude a. 0000 and 1000 have different phase,

• but same amplitude used in standard 9600 bit/s modems


- 6 -

Hierarchical Modulation• DVB-T modulates two separate data streams onto a single DVB-T

stream• High Priority (HP) embedded within a Low Priority (LP) stream• Multi carrier system, about 2000 or 8000 carriers• QPSK, 16 QAM, 64QAM• Example: 64QAM

– good reception: resolve the entire 64QAM constellation

– poor reception, mobile reception: resolve only QPSK portion

– 6 bit per QAM symbol, 2 most significant determine QPSK

– HP service coded in QPSK (2 bit), LP uses remaining 4 bit

Q

I

00

10

000010 010101


- 7 -

For the transmission of symbols with rate r a frequency bandwidth of

f ≥ r

is necessary (with appropriate pulse shaping).

For the transmission of symbols with rate r a frequency bandwidth of

f ≥ r

is necessary (with appropriate pulse shaping).

Frequency Bandwidth

information sourceinformation sourcer

information sinkinformation sink

sender

channel

receiver


- 8 -

short signal

long signal narrow frequency spectrum

wide frequency spectrum

pulse shaping

infinite length signale.g. sine

finite frequency spectrum

finite length signale.g. rectangle (“bit“)

infinite frequency spectrum

Signal length vs. Bandwidth


- 9 -

Multicarrier Modulation (MCM)• Idea

– Use of a number of carriers simultaneously for one signal/user in order to reduce ISI

• Approach– If c symbols/s are to be transmitted, distribution over n

subcarriers, each with a code rate of c/n symbols/s– Results in n carriers with lower speed and less problems

of ISI. – Frequency selective fading leads to attenuation of single

carriers only– Typical guard time between symbols– It is also possible to send identical symbols over several

carriers


- 10 -

MCM – OFDM• OFDM is a special implementation of MCM with orthogonal

subcarriers + very efficient FFT based algorithms for modulation/demodulation

• Orthogonal subcarriers:

• Symbol rate 1/T for each subcarrier is selected in a way, that they equal the next carrier with separation Δf, thus subcarriers are orthogonal irrespective of φ.

,...2,1,/:with

0)2sin()2sin(

==−

=++∫nTnff

tftf

jk

T

o

jjkk ϕπϕπ


- 11 -

OFDM Implementation• MC Modulator generates k independent QAM subcarriers,

each with a symbolrate 1/T.• Simplification: use of iFFT (Inverse Fast Fourier Transform)

for modulation and FFT for demodulation


- 12 -

Structure of an OFDM communication system

Receiver


- 13 -

Application of OFDM• ADSL and VDSL broadband access via telephone lines • All recent WLAN standards, e.g. IEEE 802.11a, g, n, ad• Digital audio broadcasting systems: EUREKA 147, Digital

Radio Mondiale, HD Radio, T-DMB and ISDB-TSB• Terrestrial digital TV systems DVB-T, DVB-H, T-DMB and

ISDB-T. • IEEE 802.16 or WiMax Wireless MAN standard• Flash-OFDM cellular system. • Ultra wideband (UWB) systems • Power line communication (PLC)

Source Wikipedia, Nov. 06


- 14 -

OFDM Key FeaturesAdvantages

– Can easily be adopted to severe channel conditions without complex equalization

– Robust to narrow-band co-channel interference– Robust to inter-symbol interference and fading caused by multipath

propagation– High spectral efficiency– Efficient implementation by FFTs– Low sensitivity to time synchronization errors– Tuned sub-channel receiver filters are not required (unlike in

conventional FDM)– Facilitates Single Frequency Networks, i.e. transmitter

macrodiversity.Disadvantages

– Sensitive to Doppler shift– Sensitive to frequency synchronization problems– Inefficient transmitter power consumption, since linear power

amplifier is required.Source: Wikipedia, Nov. 06


- 15 -

OFDM in IEEE 802.11a (and HiperLAN2)• OFDM with 52 used subcarriers (64 in total)• 48 data + 4 pilots• 312.5 kHz spacing

subcarriernumber

1 7 21 26-26 -21 -7 -1

channel center frequency

312.5 kHzpilot

From: Schiller, Mobilkommunikation


Error recovery• Forward Error Correction (FEC)

― Added redundancy to correct transmission errors at the receiver channel coding→― errors after decoding occur if error-correction capability of the code

is passed― No feedback channel required― Channel condition affects the quality of data transmission

→ Varying reliability, constant bit throughput• Automatic Repeat Request (ARQ)

― Small amout of redundancy is added to detect transmission errors― Retransmission of data in case of detected error― Feedback channel required― Channel condition affects throughput

→ constant reliability, but varying throughput• Hybrid FEC/ARQ


- 17 -

Principle of Channel Coding

Encoder: device that maps information word u onto code word cby adding redundancy ⇒ code rate R

c=k /n

• Systematic encoder: codeword x contains information word u• Non-systematic encoder: codeword x does not explicitly contain

information word u

channelencoderchannelencoder

u = [u0, u

1, ..., u

k-1]

x = [x0, x

1, ..., x

n-1]

u x

k n


- 18 -

Visualizing Distance Properties with Code Cube


- 19 -

• Linear Block Codes– k-digit information word is transformed into n-digit codeword

• Convolutional Codes– m-bit word is transformed into n-bit word, code rate m/n– transformation is a function of last k information symbols, where k

is the constraint length of the code.– Codes symbols are calculated by modulo 2 additions of memory

counters

Error Correcting Codes (1)


- 20 -

Error Correcting Codes (2)

Example: (2,1,3)-convolutional code with generators g1=7

8

and g2=5

8

Code is non-systematic and non-recursive (NSC-Code)R

c=1/2, L

c=3 → m = 2


- 21 -

Properties of Convolutional Codes

• Only a small number of simple codes is of practical interest

• not constructed by algrebraic methods but by computer (advantage of simple mathematical description)

• Easy processing of soft-decision input, compute soft-decision output (for block codes, only hard-decision decoding)

• Systematic and non-systematic encoders (mostly non-systematic codes are of practical interest)


- 22 -

Interleaving


- 23 -

Concatenated Codes

Parallel concatenated codes(turbo codes)

Serial concatenated codes


- 24 -

ARQ• David/Benkner p.235• Return Channel is required• ARQ protocols

– „Stop and Wait“• Waits until positive ack. received or timer expires

– „Go back N“• Continous transmission of data, if NACK received,

continues N steps back

– „Selective Repeat“– „Hybrid ARQ“


ARQ: General Architecture

ARQcontrol

discretechannel

ARQcontrol

Feedback channel

ACK/NAK

source sink


- 27 -

ARQ: Stop-and-Wait


- 28 -

ARQ: Go-Back-N


- 29 -

ARQ: Selective Repeat


- 30 -

ARQ Pros & Cons• Advantages

– Simple protocol– Quasi adaptive scheme, adapts to channel

properties and can therefore be very efficient• Disadvantages

– Difficult to guarantee constant end-to-end delay and constant net (user) bit rate

– If channel quality is very low, retransmission is not efficient enough (will be retransmitted and have error again)


- 31 -

Hybrid FEC/ARQ Systems


- 32 -

Spread Spectrum Technology• Problem of radio transmission: frequency dependent fading can wipe out

narrow band signals for duration of the interference• Solution: spread the narrow band signal into a broad band signal using a

special code• protection against narrow band interference

• protection against narrowband interference• Side effects:

– coexistence of several signals without dynamic coordination– tap-proof

• Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread signal

signal

spreadinterference

f f

power power


- 33 -

Effects of spreading and interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiverf

v)

user signalbroadband interferencenarrowband interference

dP/df


- 34 -

Spreading and frequency selective fading

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

22

22

2

frequency

channelquality

1

spreadspectrum

narrowband channels

spread spectrum channels


- 35 -

DSSS (Direct Sequence Spread Spectrum) (1)• XOR of the signal with pseudo-random number (chipping sequence)

– many chips per bit (e.g., 128) result in higher bandwidth of the signal

• Advantages– reduces frequency selective

fading– in cellular networks

• base stations can use the same frequency range

• several base stations can detect and recover the signal

• soft handover

• Disadvantages– precise power control necessary

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit periodtc: chip period


- 36 -

DSSS (Direct Sequence Spread Spectrum) (2)

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator


- 37 -

FHSS (Frequency Hopping Spread Spectrum) (1)• Discrete changes of carrier frequency

– sequence of frequency changes determined via pseudo random number sequence

• Two versions– Fast Hopping:

several frequencies per user bit– Slow Hopping:

several user bits per frequency

• Advantages– frequency selective fading and interference limited to short period– simple implementation– uses only small portion of spectrum at any time

• Disadvantages– not as robust as DSSS– simpler to detect


- 38 -

FHSS (Frequency Hopping Spread Spectrum) (2)

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time


- 39 -

FHSS (Frequency Hopping Spread Spectrum) (3)

modulatoruser data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

chapter 2 – overvie · arq: selective repeat. tzi – fb 1 – kommunikationsnetze andreas...

Documents